Create an upgrading guide for Jessie to Stretch

[WasabiFan]

We should look into writing a simple page that lists the changes that one needs to make to upgrade their scripts to our v2 release for Stretch. The most important change would be to imports (it's looking like we might rename the module to ev3dev2) but there are some sensor types which we also shifted around. There's an issue with a full list.

It would also be nice to outline the new features, specifically the new API surface for existing classes and the motor pairs.

[WasabiFan]

With the 2.0.0-beta1 release, this is now a thing that exists. It's available here:

https://ev3dev-lang.readthedocs.io/projects/python-ev3dev/en/ev3dev-stretch/upgrading-to-stretch.html

If you don't have the V2 library installed already, you can do so yourself:

sudo apt-get update
sudo apt-get install python3-ev3dev2